U.S. patent application number 16/305551 was filed with the patent office on 2021-07-22 for photodetector adapted to provide additional color information.
The applicant listed for this patent is BAE Systems Imaging Solutions Inc.. Invention is credited to James E. Crouch, John W. Ladd, Alberto M. Magnani.
Application Number | 20210225913 16/305551 |
Document ID | / |
Family ID | 1000005537472 |
Filed Date | 2021-07-22 |
United States Patent
Application |
20210225913 |
Kind Code |
A1 |
Ladd; John W. ; et
al. |
July 22, 2021 |
PHOTODETECTOR ADAPTED TO PROVIDE ADDITIONAL COLOR INFORMATION
Abstract
An apparatus for forming a color image of a scene and a method
for utilizing that apparatus are disclosed. The apparatus includes
a plurality of pixel sensors. Each pixel sensor includes a first
photodetector includes first main photodiode and a first floating
diffusion node. The first main photodiode is characterized by a
first light conversion efficiency as a function of wavelength of a
light signal incident thereon. The first floating diffusion node
includes a parasitic photodiode characterized by a second light
conversion efficiency as a function of the wavelength. The second
light conversion efficiency is different from the first light
conversion efficiency as a function of wavelength. A controller
generates an intensity of light in each of a plurality of
wavelength bands for the pixel sensor utilizing a measurement of
the light signal by each of the first main photodiode and the first
parasitic photodiode in that photodetector.
Inventors: |
Ladd; John W.; (Santa Clara,
CA) ; Crouch; James E.; (Santa Rosa, CA) ;
Magnani; Alberto M.; (Danville, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BAE Systems Imaging Solutions Inc. |
San Jose |
CA |
US |
|
|
Family ID: |
1000005537472 |
Appl. No.: |
16/305551 |
Filed: |
May 31, 2017 |
PCT Filed: |
May 31, 2017 |
PCT NO: |
PCT/US2017/035122 |
371 Date: |
November 29, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62343314 |
May 31, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04N 5/3575 20130101;
H01L 27/14672 20130101; H01L 27/14621 20130101; H01L 27/14609
20130101 |
International
Class: |
H01L 27/146 20060101
H01L027/146; H04N 5/357 20060101 H04N005/357 |
Claims
1. An apparatus comprising a plurality of pixel sensors, each pixel
sensor comprising a first photodetector comprising: a first main
photodiode characterized by a first light conversion efficiency as
a function of wavelength of a light signal incident thereon; and a
first floating diffusion node comprising a first parasitic
photodiode characterized by a second light conversion efficiency as
a function of said wavelength, said second light conversion
efficiency, being different from said first light conversion
efficiency as a function of wavelength, said first floating
diffusion node also being illuminated by said light signal; said
apparatus further comprising a controller that generates an
intensity of light in each of a plurality of wavelength bands for
said pixel sensor utilizing a measurement of said light signal by
each of said first main photodiode and said first parasitic
photodiode in that photodetector.
2. The apparatus of claim 1 wherein said second light conversion
efficiency is greater than three percent of said first light
conversion efficiency.
3. The apparatus of claim 1 wherein said second light conversion
efficiency is greater than ten percent of said first light
conversion efficiency.
4. The apparatus of claim 1 further comprising a plurality of
second photodetectors, each pixel sensor being associated with one
of said plurality of second photodetectors, said second
photodetectors having a second main photodiode with a third light
conversion efficiency as a function of said wavelength, said third
light conversion efficiency being different from said first and
second light conversion efficiencies, said second photodiode
providing a third measurement of said light signal, said third
measurement being used by said controller to generate said
intensities of light in said plurality of wavelength bands.
5. The apparatus of claim 4 wherein there is one second
photodetector for each pixel sensor.
6. The apparatus of claim 4 wherein there are fewer second
photodetectors then pixel sensors, at least one of said second
photodetectors being shared by a plurality of pixel sensors.
7. The apparatus of claim 6 wherein said controller interpolates
intensity measurements from a plurality of said second
photodetectors to arrive at said third measurement.
8. The apparatus of claim 4 wherein each second photodetector
further comprises a second floating diffusion node that is separate
from said first floating diffusion node, said second floating
diffusion node comprising a second parasitic photodiode, said
controller using a signal from said second parasitic photodiode to
generate said intensities in each of said plurality of wavelength
bands.
9. A method for generating a color image of a scene comprising a
plurality of color pixels, said method comprising: projecting an
image of said scene on an array of pixel sensors, each pixel sensor
generating one color pixel of said color image, each pixel sensor
comprising a first photodetector comprising: a first main
photodiode characterized by a first light conversion efficiency as
a function of wavelength of a light signal incident on said pixel
sensor; and a first floating diffusion node comprising a first
parasitic photodiode characterized by a second light conversion
efficiency as a function of said wavelength, said second light
conversion efficiency, being different from said first light
conversion efficiency as a function of wavelength, said first
floating diffusion node also being illuminated by said light
signal; generating first and second light intensity values for each
pixel sensor from said first main photodiode and said first
floating diffusion node in each of said pixel sensors; generating a
third light intensity value corresponding to each pixel sensor
utilizing a corresponding one of a plurality of second
photodetectors having a third light conversion efficiency as a
function of wavelength that is different from first and second
light conversion efficiencies; and generating each color pixel
utilizing said first, second, and third light intensity values
corresponding to that pixel.
10. The method of claim 9 wherein said second photodetector is part
of each pixel sensor.
11. The method of claim 9 wherein said plurality of second
photodetector comprises one second photodetector that is shared by
a plurality of said pixel sensors.
12. The method of claim 9 wherein intensity values from two or more
of said plurality of second photodetectors are interpolated to
arrive at said third light intensity value for one of said pixel
sensors.
13. The method of claim 9 wherein each second photodetector
comprises a second floating diffusion node comprising a second
parasitic photodiode and wherein a light intensity value from said
second parasitic photodiode is used also used to generate said
color pixel.
Description
BACKGROUND
[0001] Imaging arrays for use in color photography typically use
four photodiodes per pixel in the final image. The photodiodes are
typically arranged in a 2.times.2 array with a red filter over one
photodiode, a blue filter over a second photodiode and a green
filter over the remaining two photodiodes. The color filters select
a relatively broad range of wavelengths, and hence, the photodiodes
measure an average of the light intensity over a broad range of
wavelengths in the red, blue, and green wavelength bands. Hence,
these photodiodes are relatively insensitive to wavelength
variations within the acceptance band of the filters. In addition,
the use of four photodiodes for each image pixel increases the cost
of the imaging array over a monochrome array by a factor of
approximately four. Finally, the filters reduce the light reaching
the photodiodes, and hence, either lower the sensitivity of the
array in low light conditions or require larger photodiodes to
compensate for the loss in intensity.
SUMMARY
[0002] The present invention includes an apparatus for forming a
color image of a scene and a method for utilizing that apparatus.
The apparatus includes a plurality of pixel sensors. Each of the
pixel sensors includes a first photodetector that includes a first
main photodiode and a first floating diffusion node. The first main
photodiode is characterized by a first light conversion efficiency
as a function of wavelength of a light signal incident thereon. The
first floating diffusion node includes a parasitic photodiode
characterized by a second light conversion efficiency as a function
of the wavelength. The second light conversion efficiency is
different from the first light conversion efficiency as a function
of wavelength. The floating diffusion node also is illuminated by
the light signal. The apparatus also includes a controller that
generates an intensity of light in each of a plurality of
wavelength bands for the pixel sensor utilizing a measurement of
the light signal by each of the first main photodiode and the first
parasitic photodiode in that photodetector.
[0003] In one aspect of the invention, the second light conversion
efficiency is greater than three percent of the first light
conversion efficiency. In another aspect, the second light
conversion efficiency is greater than ten percent of the first
light conversion efficiency.
[0004] In a still further aspect, the apparatus includes a
plurality of second photodetectors, each pixel sensor being
associated with one of the plurality of second photodetectors, the
second photodetectors having a second main photodiode with a third
light conversion efficiency as a function of the wavelength, the
third light conversion efficiency being different from the first
and second light conversion efficiencies. The second photodiodes
provide a third measurement of the light signal, the third
measurement being used by the controller to generate the
intensities of light in the plurality of wavelength bands.
[0005] In one aspect, there is one second photodetector for each
pixel sensor.
[0006] In another aspect, there are fewer second photodetectors
then pixel sensors, at least one of the second photodetectors being
shared by a plurality of pixel sensors. In a still further aspect,
the controller interpolates intensity measurements from a plurality
of the second photodetectors to arrive at the third
measurement.
[0007] In another aspect, the second photodiode is part of a second
photodetector in each pixel sensor and the second photodetector
further includes a second floating diffusion node that is separate
from the first floating diffusion node, the second floating
diffusion node includes a second parasitic photodiode, the
processor using a signal from the second parasitic photodiode to
generate the intensities in each of the plurality of wavelength
bands.
[0008] The present invention also includes a method for generating
a color image of a scene that includes a plurality of color pixels.
The method includes projecting the image on an array of pixel
sensors, each pixel sensor generating one color pixel of the color
image. Each pixel sensor includes a first photodetector having a
main photodiode and a floating diffusion node. The first main
photodiode is characterized by a first light conversion efficiency
as a function of wavelength of a light signal incident on the pixel
sensor. The first floating diffusion node includes a parasitic
photodiode characterized by a second light conversion efficiency as
a function of the wavelength, the second light conversion
efficiency being different from the first light conversion
efficiency as a function of wavelength.
[0009] First and second light intensity values are generated for
each pixel sensor from the first main photodiode and the first
floating diffusion node in each of the pixel sensors. A third light
intensity value corresponding to each pixel sensor is generated
utilizing a corresponding one of a plurality of second
photodetectors having a third light conversion efficiency as a
function of wavelength that is different from first and second
light conversion efficiencies. Each color pixel is generated
utilizing the first, second, and third light intensity values
corresponding to that pixel.
[0010] In one aspect of the invention, the second photodetector is
part of each pixel sensor.
[0011] In another aspect, the plurality of second photodetectors
includes one second photodetector that is shared by a plurality of
the pixel sensors.
[0012] In another aspect, intensity values from two or more of said
plurality of second photodetectors are interpolated to arrive at
said third light intensity value for one of said pixel sensors.
[0013] In a further aspect, each second photodetector includes a
floating diffusion node that includes a second parasitic
photodiode, and a light intensity value from the second parasitic
photodiode is used also used to generate the color pixel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic drawing of a typical prior art
photodetector in one column of photodetectors in an imaging
array.
[0015] FIG. 2 illustrates a photodetector in which the parasitic
photodiode is utilized in an image measurement.
[0016] FIG. 3A illustrates a conventional imaging array.
[0017] FIG. 3B illustrates which illustrates an imaging array using
four photodetector pixel sensors according to one embodiment of the
present invention.
[0018] FIG. 3C illustrates a pixel sensor having two photodetectors
of different sizes.
[0019] FIG. 4 illustrates the ratio of the two conversion
efficiencies in one embodiment.
[0020] FIG. 5 illustrates a pixel sensor having two main
photodiodes that share a common floating diffusion node that has a
parasitic photodiode response.
DETAILED DESCRIPTION
[0021] For the purposes of the present discussion, a digital camera
converts an image of a scene into an array of pixels representing
the scene. In the case of a color image, each pixel can be
represented by a color vector having components in three or more
color channels. Each color component typically represents the
intensity of the light in a corresponding band of wavelengths.
Typically, the color components represent the intensities in the
red, blue, and green wavelength bands in the optical spectrum.
[0022] The imaging array that captures the light typically includes
a number of photodetectors that are used to measure the light
intensities that are used to generate the color vector. In the
following discussion, the group of photodetectors that provides the
information for a single pixel in the final image will be referred
to as the pixel sensor for that pixel. In conventional color
imaging detectors, there are four photodetectors in each pixel
sensor. Each photodetector is covered by a filter that selects
light in a particular band of wavelengths. In systems that use
three color components, one photodetector is covered by a filter
that passes red light and blocks other bands, one photodetector is
covered by a filter that passes blue light, and two photodetectors
are covered by filters that pass green light.
[0023] In conventional imaging arrays, each photodetector has a
single photodiode that converts the incident light on that
photodetector during an exposure period to a charge and circuitry
for generating a voltage signal from that charge. The voltage
signal is typically digitized to produce an intensity reading for
the light incident on that photodetector. In one aspect of the
present invention, each photodetector includes two photodiodes,
while requiring only the same amount of silicon surface area of a
single conventional photodetector having one photodiode. In
addition, the two photodiodes of the photodetector have different
light sensitivities as a function of wavelength of the incident
light, and hence, can be used to provide color information without
requiring wavelength filters that reduce the light intensity.
[0024] FIG. 1 is a schematic drawing of a typical prior art
photodetector in one column of photodetectors in an imaging array.
Photodetector 21 includes a photodiode 22 that measures the light
intensity at a corresponding pixel in the image. As noted above, in
prior art pixel sensors, there are typically four such
photodetectors corresponding to each pixel sensor. Initially,
photodiode 22 is reset by placing gate 25 in a conducting state and
connecting floating diffusion node 23 to a reset voltage, Vr. Gate
25 is then closed and photodiode 22 is allowed to accumulate
photoelectrons during an exposure period. A potential on an
optional anti-blooming circuit 27 sets the maximum amount of charge
that can be accumulated on photodiode 22. If more charge is
accumulated than allowed by anti-blooming circuit 27, the excess
charge is removed from photodiode 22.
[0025] After photodiode 22 has been exposed, the charge accumulated
in photodiode 22 is typically measured by noting the change in
voltage on floating diffusion node 23 when the accumulated charge
from photodiode 22 is transferred to floating diffusion node 23.
For the purposes of the present discussion, a floating diffusion
node is defined to be an electrical node that is not tied to a
power rail, or driven by another circuit. In the photodetector
shown in FIG. 1, one source/drain side of the transfer gate
transistor and the drain side of the photodetector reset transistor
are tied together. When neither gate of these transistors is
active, the connected source/drain diffusions are considered to be
one floating diffusion node. Floating diffusion node 23 is
characterized by a capacitance represented by capacitor 23'. In
practice, capacitor 23' is charged to a voltage, Vr, and isolated
by pulsing the reset line of gate 24 prior to floating diffusion
node 23 being connected to photodiode 22. The charge accumulated on
photodiode 22 is transferred to floating diffusion node 23 when
gate 25 is opened. The voltage on floating diffusion node 23 is
sufficient to remove all of this charge, leaving the voltage on
floating diffusion node 23 reduced by an amount that depends on the
amount of charge transferred and the capacitance of capacitor 23'.
Hence, by measuring the change in voltage on floating diffusion
node 23 after gate 25 is opened, the accumulated charge can be
determined.
[0026] This scheme assumes that the reset potential on the floating
diffusion node prior to transferring the charge from photodiode 22
is known. In practice, the actual reset voltage will vary from the
voltage, Vr, due to noise. If this noise is significant, a
correlated double sampling algorithm can be utilized to reduce the
errors caused by the noise. In the correlated double sampling
processing, the actual voltage on the floating diffuse node after
the reset operation is performed is measured prior to connecting
photodiode 22 to floating diffusion node 23. The difference between
this measured reset voltage and the voltage after the charge has
been transferred is used to calculate the charge generated during
the exposure. The procedure starts by connecting floating diffusion
node 23 to Vr using reset gate 24. The potential on floating
diffusion node 23 is then measured by connecting source follower 26
to readout line 31 by applying a select signal to word line 28.
This reset potential is stored in column amplifier 32. Next, gate
25 is placed in a conducting state and the charge accumulated in
photodiode 22 is transferred to floating diffusion node 23. It
should be noted that floating diffusion node 23 is effectively a
capacitor that has been charged to Vr. Hence, the charge leaving
photodiode 22 lowers the voltage on floating diffusion node 23 by
an amount that depends on the capacitance of floating diffusion
node 23 and the amount of charge that is transferred. The voltage
on floating diffusion node 23 is again measured after the transfer.
The difference in voltage is then used to compute the amount of
charge that accumulated during the exposure.
[0027] A "parasitic photodiode" is an inherent property of the
floating diffusion node in many photodetector designs. A depletion
region is formed by the floating diffusion node and the gates of
other transistors in the photodetector. The parasitic photodiode
collects photoelectrons from photons that are converted in the
silicon under the floating diffusion node. In the prior art, care
is taken to minimize the conversion efficiency of the parasitic
photodiode. For example, in some designs, a mask is provided over
the floating diffusion node to block light from striking the node
to reduce the light conversion efficiency of the parasitic
photodiode. In contrast, the present invention is based on the
observation that a photodetector of the type discussed above can be
modified to enhance the light conversion efficiency of the
parasitic photodiode to provide a second photodiode that does not
significantly increase the size of the photodetector, and hence,
the present invention provides the advantages of a two-photodiode
photodetector in which the two photodiodes have different color
responses without significantly increasing the photodetector.
[0028] Refer now to FIG. 2, which illustrates a photodetector in
which the parasitic photodiode is utilized in an image measurement.
To distinguish the parasitic photodiode 42, photodiode 22 will be
referred to as the "main photodiode". To simplify the following
discussion, those elements of photodetector 21 that serve functions
analogous to those discussed above with respect to FIG. 1 have been
given the same numeric designations and will not be discussed
further unless such discussion is necessary to illustrate a new
manner in which those elements are utilized. In general, parasitic
photodiode 42 that is part of floating diffusion node 43 has a
detection efficiency that is significantly less than that of main
photodiode 22. For the purposes of the present discussion, the
detection efficiency of a photodiode is a function of the
wavelength of the light that irradiates that photodiode and is
defined to be the average number of photoelectrons generated per
unit of energy of photons of a particular wavelength. In co-pending
U.S. patent application Ser. No. 14/591,873, filed on Jan. 7, 2015,
this difference in detection efficiency between the main and
parasitic photodiodes is utilized to increase the dynamic range of
the photodetector by adjusting the conversion efficiencies of the
two photodiodes such that the parasitic photodiode provides an
intensity measurement at incident light intensities that are too
great to be measured in the main photodiode. For example, the
parasitic photodiode could have a conversion efficiency that is
three percent of the conversion efficiency of the main photodiode.
In another example the parasitic photodiode has a conversion
efficiency that is greater than ten percent of the conversion
efficiency of the main photodiode. Other examples of the detection
efficiency and methods for adjusting the relative conversion
efficiencies of the main and parasitic photodiodes are discussed in
this US Patent Application, which is incorporated by reference
herein, and hence, will not be discussed in detail here.
[0029] The present system is based on the observation that the
parasitic photodiode has a different color response than the main
photodiode in the photodetector. That is, the ratio of the
conversion efficiency of the parasitic photodiode to the conversion
efficiency of the main photodiode is not a constant as a function
of the wavelength of the incident light. In fact, as will be
discussed in more detail below, this ratio can be used to provide
information on the average wavelength of the light incident on the
two photodiodes. This information can be used to refine the color
measurement without the need to provide a color filter over the
photodetectors. This information can also be used to reduce the
number of photodetectors per pixel sensor in the imaging array. In
one aspect of the present system, controller 52 includes a
processor that performs the color computations that generate the
intensities in the color channels utilizing the parasitic
photodiode as one photodetector in the pixel sensor.
[0030] The number of photoelectrons generated by a photon
interacting with the photodiode as a function of the photon
wavelength depends on a number of factors. The main photodiode may
have a significantly different depth profile than the parasitic
photodiode, hence, the wavelength response of the two different
photodiodes will be different. In addition, the main photodiode is
typically a pinned photodiode and the parasitic photodiode is not.
Other factors affecting the relative parasitic response include a
difference in the spatial position of the parasitic photodiode
relative to the micro-lens that is placed over the pixel detector,
and the difference in electrical and optical crosstalk as a result
of a number of geometric, layout, implant, and structural
differences. In general, it is advantageous to adjust the overall
conversion efficiency of the parasitic photodiode to be close to
that of the main photodiode; however, in practice the conversion
efficiency of the parasitic photodiode is less than that of the
main photodiode. The ratio of the two conversion efficiencies in
one embodiment of the present invention is shown in FIG. 4.
[0031] Denote the signal from the main photodiode by S.sub.m and
the signal from the parasitic photodiode by S.sub.p. Then,
S.sub.m=I.sub.rC.sub.mr+I.sub.gC.sub.mg+I.sub.bC.sub.mb
S.sub.p=I.sub.rC.sub.pr+I.sub.gC.sub.pg+I.sub.bC.sub.pb (1)
Here, C.sub.mr, C.sub.mg, and C.sub.mb are calibration constants
for the main photodiode that are related to the conversion
efficiency of the main photodiode in the red, green, and blue wave
bands, respectively, and C.sub.pr, C.sub.pg, and C.sub.pb are
calibration constants for the parasitic photodiode that are related
to the conversion efficiency of the parasitic photodiode in the
red, green, and blue wave bands, respectively. Eqs. (1) provide two
equations in three unknowns, i.e., the intensity of the incident
light in the three wavelength bands. Hence, these equations cannot
be solved to provide the desired intensities without some
additional information in the general case.
[0032] Refer now to FIG. 3A which illustrates a conventional
imaging array. Each pixel sensor 81 has four photodetectors. It is
assumed that the light intensity does not vary significantly over
the pixel, and hence, each photodetector receives the same incident
light intensity. Typically, each photodetector has a filter that
limits the light reaching the photodetector to light in a
particular wavelength band. The filters are denoted by R, B, and G
in the figure. Typically, two photodetectors are used to measure
the intensity in the green wavelengths in the Bayer RGB color
filter pattern scheme shown in the figure. As noted above, the
filters remove a significant fraction of the light, and hence, the
shot noise associated with each photodetector is increased relative
to the shot noise that would be experienced without the filters. In
essence, this prior art pixel design provides four intensity
measurements from which to obtain the intensity of light in the
three wavelength bands.
[0033] Refer now to FIG. 3B, which illustrates an imaging array
using four photodetector pixel sensors according to one embodiment
of the present invention. Each pixel sensor 82 now includes one
photodetector 83 that has a color filter, F, and three
photodetectors 84 that are unfiltered. Since each photodetector now
provides two intensity measurements having different color
weightings, there are effectively eight useful intensity
measurements. Suppose a filter is provided over one of the
photodetectors to provide a third response. The remaining
photodetectors do not include filters. Then,
S.sub.i,m=I.sub.rC.sub.mr+I.sub.gC.sub.mg+I.sub.bC.sub.mb
S.sub.i,p=I.sub.rC.sub.pr+I.sub.gC.sub.pg+I.sub.bC.sub.pb
S.sub.fm=I.sub.rC.sub.fmr+I.sub.gC.sub.fmg+I.sub.bC.sub.fmb
S.sub.fp=I.sub.rC.sub.fpr+I.sub.gC.sub.fpg+I.sub.bC.sub.fpb (2)
where i runs from 1 to 3. Here, C.sub.fmr, C.sub.fmg, and C.sub.fmb
are conversion efficiencies for the main photodiode in the filtered
pixel, and C.sub.fpr, C.sub.fpg, and C.sub.fpb are conversion
efficiencies for the parasitic photodiode in the filtered pixel. A
least squares fit to the eight measurements can now be done to
provide an improved estimate of the intensities of the incident
light in the three color bands. The filter could select a
particular color channel such as a green, red, or blue filter.
Alternatively, one or more filters could merely partially attenuate
light in one of the filter channels to provide the required
independent measurement while sacrificing less light.
[0034] In principle, a pixel sensor could be utilized that has only
two photodetectors, one with a filter and one without. This would
provide four intensity measurements to determine the color
intensities. Refer now to FIG. 3C, which illustrates a pixel sensor
having two photodetectors of different sizes. Photodetector 85
lacks a filter, and photodetector 86 has a filter. The two
photodetectors provide four intensity measurements that can be used
to determine the three color intensities. The total area of the
pixel for a given signal-to-noise level will be less than that of
the conventional pixel sensor for the same signal-to-noise level,
as the amount of light received per unit area is greater due to the
lack of a filter in photodetector 85.
[0035] The above-described embodiments utilize a three-channel
color representation system to determine the intensities in
channels corresponding to the red, blue, and green wavelength
bands. Even a two-photodiode pixel sensor embodiment according to
the present invention provides more intensity measurements than
needed to determine the three channel intensities. The additional
measurements can be used to reduce noise or to provide intensities
in other color schemes having more color channels.
[0036] In the above embodiments, a second photodetector that
provides the information needed to uniquely solve for the three or
more color channel intensities is included in every pixel. However,
the response from a third photodetector that has a different light
conversion efficiency as a function of wavelength than that
provided by the main photodiode and a parasitic photodiode in a
single photodetector can be provided in other configurations. Refer
now to FIG. 5, which illustrates a pixel sensor having two main
photodiodes that share a common floating diffusion node that has a
parasitic photodiode response. To simplify the following
discussion, those elements of pixel sensor 73 that provide the same
functions as corresponding elements in photodetector 41, discussed
above with reference to FIG. 2, have been given the same numeric
designations. Pixel sensor 73 includes a second main photodiode 71
that can be connected to floating diffusion node 43 by gate 72. The
second main photodiode 71 has a different color response than
photodiode 22 and parasitic photodiode 42. For example, second main
photodiode 71 could be covered by a color filter. For example,
second main photodiode 71 could be filtered such that second main
photodiode 71 detects light preferentially in the blue or green
portions of the spectrum. The cost of adding the second photodiode
to pixel sensor 73 is much smaller than providing a complete second
photodetector to implement a pixel sensor, since second main
photodiode 71 shares all of the readout and reset circuitry. If the
intensity of light is provided by main photodiode 22, then second
main photodiode 71 need not be as large as photodiode 22, since the
purpose of second main photodiode 71 is to provide the remaining
color information, and the human eye is less sensitive to errors in
color than errors in intensity. Controller 74 combines the color
information form the three photodiodes to provide the intensities
in the three color channels.
[0037] For the purposes of the present discussion, this second main
photodetector that provides the third color equation, will be
referred to as the secondary photodetector in the following
discussion. If the incident light on a pixel were of a single pure
color, i.e., a spectrum consisting of light at one wavelength or
narrow band of wavelengths, the secondary photodetector would not
be needed to determine the wavelength. It can be shown that the
ratio of the signals from the parasitic photodiode to the main
photodiode is a function of the average wavelength of the incident
light on the photodetector. The function in question depends on the
details of the main and parasitic photodiodes and can be calibrated
for any particular design, provided the ratio of light response of
the main photodiode to the secondary photodiode is a monotonic
function of wavelength in the region of the spectrum of interest.
In one embodiment, the ratio of light response is a linear function
of the average wavelength.
[0038] The ratio of the parasitic photodiode response to the main
photodiode response is shown in FIG. 4 for one exemplary
photodetector. In this embodiment, this ratio is a linear function
of the average wavelength of the light incident on the
photodetector over a significant fraction of the optical spectrum.
The functional relationship between the ratio of the responses and
the wavelength of the incident light depends on the details of the
construction of the photodiode. If there is only one wavelength in
the incident light, this ratio can be used to determine that
wavelength and one of the equations in Eqs. (1) can be used to
determine the intensity in the one RGB triplet color that is
non-zero. It should be noted that if the average wavelength
corresponds to red or blue, and camera filters remove wavelengths
above and below the visual range, then there is only one wavelength
in the incident light, and hence, the two equations can provide a
unique solution for the wavelength. If, however, the average
wavelength is within the intermediate wavelength band, the incident
light could be the combination of two spectral lines, one in the
red and one in the blue. In this case, information from a second
photodiode or photodetector is needed.
[0039] If the information in one of the color channels changes
slowly over the image, the density of secondary photodetectors can
be reduced in the array. In this case, one secondary photodetector
can be provided for every N pixel sensors, where N>1. The
results from these sparse secondary photodetectors can be
interpolated to provide a value at each pixel detector.
[0040] The above-described embodiments of the present invention
have been provided to illustrate various aspects of the invention.
However, it is to be understood that different aspects of the
present invention that are shown in different specific embodiments
can be combined to provide other embodiments of the present
invention. In addition, various modifications to the present
invention will become apparent from the foregoing description and
accompanying drawings. Accordingly, the present invention is to be
limited solely by the scope of the following claims.
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